Emissions of transport-related pollutants have been a leading force driving progress in industry for nearly thirty years. The progressive increase in the severity of emission limits for the four regulated pollutants (CO, HC, NOx, particles) has allowed a significant improvement in air quality in particular in large urban areas.
The ever increasing use of motor vehicles requires continued efforts for a further reduction in these pollutant emissions. Reduced tolerance for European emission thresholds is expected in 2014 as part of steps for the entry into force of the Euro6 standard. Such measures set out to reduce local pollution. To have available highly efficient depolluting technologies under all driving conditions is therefore a major challenge for the transport industry. Within this context, the reduction of lean mixture nitrogen oxides (NOx) i.e. comprising excess oxygen is a major challenge associated with complex issues.
In addition, fuel consumption, having a direct link with CO2 emissions, has become a major concern for the automobile industry within a few years. Regulations were introduced in Europe after 2012 on CO2 emissions of private vehicles. It is henceforth accepted that this limit will be regularly lowered over the coming decades. The reduction of CO2 emissions has therefore taken hold as the new driving force for growth of the entire transport industry. This twofold problem of reducing local pollution (NOx) and reducing fuel consumption (CO2) is particularly difficult for Diesel engines for which the lean mix combustion is accompanied by NOx emissions difficult to treat.
Within this context the post-treatment technology SCR (“Selective Catalytic Reduction”) is used both for private vehicles and for goods vehicles. It is thus possible to focus engine operation on optimal yield, the strong NOx emissions subsequently being treated in the exhaust by the SCR system allowing highly efficient NOx reduction.
To allow the implementing of SCR technology, a reducing agent must be placed on board the vehicle for the reduction of nitrogen oxides. The system currently chosen for heavy goods vehicles uses urea in an aqueous solution as reducing agent. When injected into the exhaust, the urea decomposes to ammonia (NH3) under the effect of the temperature of the exhaust gases and allows NOx reduction on a specific catalyst. One adopted, standardised urea solution for the functioning of current SCR systems in series is referenced AUS32 (the trade name in Europe being Adblue®).
This most efficient method suffers from a certain number of drawbacks however. It has limited efficacy under cold conditions, whereas this type of situation occurs in several cases in particular for city buses. The urea tank is of large volume and weight typically 15 to 30 L for a private car, 40 to 80 L for a heavy goods vehicle. Such volume leads to complex integration in the vehicle, all the more so for a small vehicle. The depollution cost is therefore high and the excess weight is detrimental to the vehicle's fuel consumption and hence to CO2 emissions.
The option of storing ammonia in gaseous form under pressure has numerous disadvantages in terms of compactness and operating safety, and various alternative storage methods have been developed. In one of these methods the gas is absorbed inside a material e.g. salt. The storage of ammonia is then obtained within the salt through the formation of a chemical complex of ammoniacate type. The storage of ammonia in the form of absorbed gas has the advantage of a gain in volume compared with an aqueous solution, an increase in cold condition efficacy and greater compactness of the mixing point with exhaust gases in particular.
Nevertheless the implementing of this technology involves a certain number of difficulties such as:                The delivery of the ammonia stored in the matrix and intended to be injected into the exhaust is obtained under heating, most often electric heating. The extent of said heating is designed so that it is possible to reach a sufficient temperature to attain a saturation pressure of the gas (which corresponds to the material used) sufficient to ensure injection into the exhaust. Typically, it is sought to reach ammonia pressures of between 2 and 3 absolute bars at the orifice connecting the inside of the cartridge to the metering system. Once this temperature is reached, there is a time interval before obtaining ammonia ready for injection, a time interval which is dependent first on the desorption enthalpy of the gas (chemical time) and secondly on the diffusion time of the gas through the porous medium formed by the storage matrix. One means to shorten this time interval, for compatibility with motor vehicle specifications, is to boost the electric heating power to the detriment of fuel consumption and hence of the CO2 impact of the technology.        Another limitation to the system lies in the fact that once the outward flow of ammonia (towards the exhaust) is ensured, the maintaining of this flow at a sufficient level (to ensure stoichiometry with the flow of NOx to be removed) will be limited by the difference between the outward injection rate of the ammonia and the re-feeding of the void areas (porosity of the storage material), this re-feeding being limited by desorption enthalpy and resistance to diffusion of the porous medium (substantiated by characteristic head loss of flow in a porous medium). It is to be noted that the time of this maintained gas flow will be longer the more the approval standard for vehicles regarding NOx emissions will tend towards measurement under real driving emissions (RDE), contrary to measurements under dynamometer testing conditions using a specific driving cycle.        The two previously mentioned limits to the proper functioning of the system relate to the delivery of the gas from its “condensed” state inside the storage material to the dispensing and feed system directed towards the exhaust. Conversely, when the cartridge is empty it is dismounted exchanged for a full cartridge and sent for refill to a station provided for this purpose. The refill time is an important parameter of the cost of the operation.        
It is noted that in each of the above-mentioned cases, the compacting of the ammonia storage elements, nevertheless favourable for a reduction in the space taken up by a given quantity of ammonia, is obtained to the detriment of the transfer time of the ammonia from the core of the cartridge towards the injection network, and the transfer time from the ammonia filling station towards the core of the storage matrix. The present invention sets out to facilitate the dispensing of ammonia to every region of the storage matrix.